The present invention relates to combustors and, more particularly, to combustors using gaseous fuel and in which the gaseous fuel is premixed with air in a primary and a secondary zone. Combustion takes place only in the secondary zone.
Legal and environmental requirements mandate strict limits on NOx emissions of power-generation equipment. It is known that low combustion temperatures are required to attain low NOx emissions in gas turbines. The prior art contains relatively extreme measures to attain low NOx emissions. For example, it is conventional to add steam or water to the combustors of a gas turbine engine to reduce the reaction temperature therein. Although at least partially effective in reducing combustion temperature, the use of steam or water adds to the complexity of metering the proper amount thereof to the combustor, and reduces the combustor efficiency.
A further possibility includes selective catalytic reduction using a body of catalyst to encourage combustion at lower temperatures than are otherwise possible. In general, catalysts are expensive, and techniques for attaining efficient complete reaction with them are complex.
A still further possibility, and the one to which the present invention is directed, includes precisely premixing gaseous fuel and air prior to combustion. This permits combustion to take place at a lower temperature with consequent reduced NOx and CO emissions.
Extreme precision in the proportioning of gas fuel and air is required to attain the desired low emissions. The problem is compounded in a combustor wherein a primary mixing zone receives air and a portion of the fuel gas, and a secondary mixing zone receives air and the remainder of the fuel gas. Combustion takes place only in the secondary zone. Accurate metering of fuel gas is required in both the primary and secondary zones. In addition, precise proportioning of gas fuel between the primary and secondary zones is critical to low emissions. Errors in proportioning in the range of a few percent are sufficient to produce unacceptable levels of emissions. Such accuracies are difficult to attain with conventional techniques.